When was the last time you cleaned your hummingbird feeder? If it's hot out and you filled it more than a day or two ago, the nectar may have already turned.
Sugar water is mold and bacteria's favorite meal, and heat is the accelerant. Above about 85 degrees, nectar starts to ferment and grow mold in a day or two, faster in direct sun.
The feeder you filled on a Sunday can be a petri dish by Tuesday. And spoiled nectar gives hummingbirds mouth and throat infections that can kill them.
So in this heat, here's the routine: Change the nectar every one to two days, daily if it's over 90.
Dump the old, rinse the feeder with hot water, refill. If the liquid ever looks cloudy, that's fermentation starting, so pour it out immediately. Hang the feeder in shade, not blazing sun, and if the birds aren't draining it fast, switch to a smaller one or fill it halfway so less goes to waste.
And the recipe, while you're in there: one part plain white sugar to four parts water. No red dye, no honey.